There have widely been known grinding machines for grinding workpieces, which are set on respective chuck top surfaces, by moving rotating respective grinding wheels in relation to the workpieces. In such a conventional grinding machine, a very-slowly rotating grinding wheel is brought into contact with a workpiece by gradually lowering (along the Z-axis) the shaft of the grinding wheel manually, as shown in FIG. 6, and the point of contact of the grinding wheel with the workpiece is taken as zero point (whether or not the grinding wheel and the workpiece are in contact with each other is judged by the operator's feeling) for determining the relative coordinates of the grinding wheel and the workpiece. On the basis of the zero point, the depth of cut to be made in the workpiece, for example, is decided (set).
In another conventional grinding machine, a grinding wheel is brought into contact with a workpiece by gradually lowering (along the Z-axis) the shaft of the grinding wheel automatically with a predetermined voltage applied between the grinding wheel and the workpiece when the grinding wheel and the workpiece are electrically conductive, and the point of contact of the grinding wheel with the workpiece is taken as zero point (whether or not the grinding wheel and the workpiece are in contact with each other is judged by whether an electric current flows or not) for determining the relative coordinates of the grinding wheel and the workpiece.